1. Field of the Invention
This invention relates to electrical apparatus for detecting an overcurrent flowing to a conductor and for protecting the conductor from the overcurrent, and more particularly to a solid state overcurrent relay for monitoring the current flowing to a trolley wire in an underground mine and opening the circuit to the trolley wire upon the occurrence of an overcurrent to prevent the trolley wire from drawing the overcurrent.
2. Description of the Prior Art
In the operation of electrically powered vehicles, such as portal buses, locomotives, haulage vehicles and the like, propelled on tracks in an underground mine direct current power is supplied to the vehicle from an overhead trolley wire suspended from the mine roof. Direct current is conveyed through the trolley wire from a remote source. The supply of direct current to the trolley wire in an underground mine presents a continual threat of fire in the mine caused by an overcurrent flowing through the trolley wire due to a fault. The overcurrent is capable of igniting coal and other combustible materials present within the mine.
It is the conventional practice to protect trolley wires and other conductors in an underground mine from the dangerous effects of an overcurrent with standard circuit breakers. Each standard circuit breaker has an electromechanical overcurrent relay connected so that rectified current from an A.C. power source flows through the normally closed contacts and the overcurrent relay to the trolley wire and the load. The circuit breaker operating coil is normally maintained energized, and therefore the circuit breaker contacts are normally closed permitting current to flow from the power source to the load as above described. In this event a current greater than the rated current flows through the overcurrent relay, the overcurrent relay removes power from the circuit breaker operating coil. The circuit breaker contacts are then opened and current flow through the circuit breaker is terminated.
The major disadvantage of the above overcurrent protection device is the inability to test the device without drawing an overcurrent through the circuit breaker. High energy testing of this type can be hazardous to the operating personnel. Furthermore, should the overcurrent relay be ineffective due to a malfunction of one of its components, there is no automatic means to indicate the failure of the relay. Consequently, the trolley wire may continue to draw an overcurrent until the circuit breaker is manually opened or the trolley wire becomes overheated, burns, and breaks with potentially disastrous effects.
It has been proposed by other prior art systems to connect a shunt in the circuit so that current from the power source flows through the circuit breaker contacts, the shunt, the trolley and then to the load. The shunt generates a voltage signal proportional to the current drawn by the trolley wire. A solid state overcurrent relay receives an input signal from the shunt and compares it to a reference signal having a magnitude representing the maximum level of current permitted to flow through the conductor. If the reference signal exceeds the input signal, the solid state overcurrent relay permits the circuit breaker operating coil to remain energized. Therefore current is permitted to flow through the circuit breaker contacts to the load as outlined above. Upon the occurrence of an overcurrent the input signal exceeds the reference signal, and the solid state overcurrent relay deenergizes the circuit breaker operating coil. The circuit breaker contacts are then opened and current flow through the circuit breaker is terminated. Solid state relays of this type have achieved reliability by either using quality control techniques or redundant components and circuits. However, the use of quality control techniques cannot insure continued safe operation in the event of a single component failure. Failure analysis of this type of equipment has shown that single component failures are most likely to occur when (1) integrated circuits, semiconductors, capacitors, transformers, inductors and surge suppression devices fail to open circuit or short circuit and (2) resistors, lamps and relay coils also fail to open circuit. Because of the critical nature of overcurrent protection apparatus and since the units are not installed at a normally attended location, it has been determined that the use of redundant components or circuits cannot provide adequate protection in the event of one of the above single component failures.
There is need for apparatus to protect underground trolley wires and other conductors in a mine in which the apparatus is capable of being tested without drawing an overcurrent through the apparatus. There is also need that this apparatus be designed so that in the event of any of the above single component failures it will either trip the circuit breaker immediately or be able to trip the circuit breaker if an overcurrent signal is received from the shunt. With this arrangement the use of redundant components or circuitry to protect against single component failures can be avoided.